I soon became frustrated trying to replicate the graph data call of the Fitbit website. I searched again for easier and better solutions, and I ended up on this Fitbit community post, explaining a simplified way for a user to collect all his data via the official APIs. Briefly speaking you have to create a Fitbit app on the website, configure it as described in the previous link and implement a OAuth flow in order to obtain the access tokens needed for the API calls.
If you use Python, I suggest to rely on the official python implementation of the Fitbit API and give a look here for a clear explanation on what needs to be done to have all setup and running.
I have to admit that this ended up to be easier and cleaner that the previous scraper solution, still, you need to keep a couple of things in mind:

1. You have to write some code to clean the JSON data returned by the API, and extract the info relevant to you
2. Seems like the API have a limit of 150 calls per hour, so you might need to repeat the operation several times depending on how much and what data you need